The propulsion system for large cargo ships, for example container ships, normally provides one slow-running diesel engine for each ship propeller. This slow-running diesel engine supplies all the propulsion power for this propeller. The propulsion power of the diesel engine starts at about 10 to 15 MW. The diesel engines that are used operate on the two-stroke principle. The high power that is required and the low rated rotation speed result in a large swept volume, and thus a large physical volume with a large mass.
A correspondingly large amount of space must be provided in the ship's hull for the diesel engine or, for a multishaft propulsion system, the diesel engines, to be precise at a point which is located close to the stern with respect to the overall length. The engine room is located at a point which intrinsically would be particularly highly suitable for being loaded with containers. There is also a large amount of unused dead space.
Because of the heavy weight of the diesel engine, the hull must be reinforced to a considerable extent at the relevant points.
As in the case of all internal combustion engines, the relative fuel consumption also varies in the case of ship diesel engines as a function of the operating point, that is to say of the rotation speed. The rotation speed range which is best with regard to the fuel consumption occurs at about 85 to 90% of the rated rotation speed for which the diesel engine is designed. At lower rotation speeds, the fuel consumption can be even worse. The ship's captain must therefore keep the propulsion system as close as possible to this optimum operating point.
Variable-pitch propellers, in which the angle of the blades of the ship propeller can be controlled as a function of the torque, provide a certain capability to vary the operation point in the direction of better fuel consumption, even in poor load situations. Nevertheless, the optimum operating point cannot be achieved in all movement conditions.
In addition to the poor fuel consumption at rotation speeds below the rated rotation speed, considerable thermal loads also occur at low rotation speeds, such as those which are required in the lower speed of motion range when the ship is being accelerated. In order to accelerate the ship, the diesel engine has to emit a high torque at a low rotation speed, that is to say it runs at a low rotation speed with a high cylinder charge. The thermal load on the cylinder heads is correspondingly high.
Despite all these disadvantages, the ship diesel engine in container ships is used virtually exclusively, since its procurement costs are very low. The diesel engine is robust and can be produced in countries with low wages, which reduces the production costs.
Besides the diesel drive for ships, it is also known from practice for electric motors to be used for driving the ship propeller. The electrical power for the propeller motor is produced by means of a diesel generator system. High-speed four-stroke diesel engines can be used in this case, which have a considerably better power-to-weight ratio than slow-running diesel engines. Their rotation speed is higher by a factor of approximately 4 to 10 than the rotation speed of the diesel engine for the ship propeller.
Furthermore, converters based on semiconductors can be used to always operate the diesel engine for the diesel generator system independently of the load at that rotation speed at which the fuel consumption with respect to the output power is optimum. This propulsion system concept makes it possible to avoid thermal problems resulting from low rotation speeds and high cylinder charges.
The considerable advantages of an electrical propulsion system are countered by comparatively high investment costs, which are considerably greater than the costs involved for a diesel engine that drives the ship propeller directly.
Finally, a mixed propulsion system is known from practice, which provides a slow-running diesel engine for the ship propeller as the main propulsion device. In addition, a comparatively small electric motor is coupled to the propeller shaft, as a booster. The electric motor power is at most 10% of that of the diesel engine.
The booster motor is used in particular in the low rotation speed range in order to avoid thermally overloading the main propulsion system diesel. A further purpose of the booster motor is to improve the control system dynamic response, for example in operating situations when the propeller is partially moving in and out of the water owing to heavy sea states.